The invention relates generally to metallurgical processes. More specifically, it is directed to treating processes for metal-based substrates.
A variety of specially-formulated coatings is often used to protect metal parts that are exposed to high temperatures, e.g., metal parts made from superalloys. For example, aluminide coatings are often used to provide oxidation- and corrosion-resistance to superalloys, which can serve as a bond layer between the superalloy substrate and a thermal barrier coating (TBC).
In one process for depositing an aluminide coating, a very thin layer of platinum (e.g., about 1-6 microns) is first applied to the substrate surface by electroplating, and an aluminide material is then applied by a vapor deposition process. The aluminum reacts with the platinum and with the substrate material (e.g., nickel) to form a variety of intermetallic compounds, such as platinum aluminide and nickel aluminide. Upon exposure to oxidation, an aluminum oxide (alumina) film forms on the surface of the aluminide, which serves as a barrier against further reactions with environmental constituents, thereby maintaining the integrity of the substrate.
It is sometimes necessary to repair the aluminide coating. For example, coatings applied on turbine engine parts are frequently repaired when the turbine itself is overhauled. The repair process can involve various steps, including stripping of the aluminide coating, and deposition of a new aluminide coating in the affected area. In current practice, the aluminide materials are often stripped from the substrate by exposure to an acid, such as hydrochloric acid, nitric acid, or phosphoric acid.
The present inventors have recognized drawbacks associated with the use of the various stripping compositions mentioned above. Frequently, the overall procedure is time-consuming, requiring as much as 4-6 hours of contact time with the stripping compositions and with rinsing solutions. Moreover, some of the stripping compositions do not remove sufficient amounts of the aluminide material, and further time and effort are required to complete the removal. Moreover, some of the compositions have low selectivity, as demonstrated by attacking the base metal of the substrate, pitting the base metal substrate or damaging the metal via intergranular boundary attack.
Furthermore, many of the currently-used stripping compositions have to be used at elevated temperatures, e.g., above about 77xc2x0 C. Operation at these temperatures can attack masking materials that are used to protect selected portions of the part, e.g., airfoil roots or internal surfaces, while also raising energy costs and potentially-requiring additional safety precautions.
Moreover, some of the prior art processes require heavy grit-blasting prior to treatment, to roughen the substrate surface, and after exposure to the stripping compositions. These steps can be time-consuming, and can also damage the substrate, thereby limiting part life.
It is thus apparent that new processes for removing aluminide-based materials from metal substrates would be welcome in the art.
The present invention relates to methods for removing an aluminide material from a substrate. According to an embodiment of the present invention, a method includes the steps of contacting the surface of a substrate with at least one stripping composition to degrade the coating, wherein the stripping composition is selected from the group consisting of(i) aliphatic or aromatic sulfonic acids; (ii) a solution of an inorganic acid and an organic solvent; and (iii) sulfuric acid or an aqueous solution of sulfuric acid; and (b) removing the degraded coating.
According to an embodiment described in more detail hereinbelow, stripping composition (i), including an aliphatic or aromatic sulfonic acid, further includes an inorganic or organic additive.
Other details regarding the various embodiments of this invention are provided below.